Gas purification



Patented June 9, 1953 .,Ko'erner E..Leutz Toledo, Ohio, assignor to SunOil Gompany,.P-hiladelphia, Pa-.,- a-corporation of New J 11 Glaims.

I "This invention relateszto the removal of ny-- drogenxsulfideiromgaseous mixtures. :Moreqiartioularly, it relates to the selectiveremoval :of hydrogen sulfide from gaseous mixtures :containing the sameand carbon dioxide.

Removal of -hydrogen sulfide "from gaseous mixtures has previously :beenaccomplished by contacting ssuchzmixtures with 'rmetalliezoxides solidform or with alkaline solutions aor suspen sions. The use of solid:m'asses has :the :dtsad vantage that; :arter such masses "have Ibeeomefouled with hydrogenrsulfide, they are .tdifiicult :to

. regenerate' ror'iturther use. Theme of alkaline aqueous materials has@the disadvantage that such materials do not selectively remove hydrogensulfide from gaseous material's :eontaining carbon dioxideas well ashydrogen sulfide. Such materials remove carbon-dioxide :too, with theresult that -more of the material is "required for 'a given amount :of:gas.

It is often the .case that iindustrial .gases soontain' varyingquantities :of carbon dioxide tas well as hydrogen sulfide. Anexamples-is refineryihy :droearbon gases such -:as are derived x-fmm:cata- .lytic .eraoking -:operations. While .it .is desirable .to removehydrogen sulfide efrom :these .gaseous vmixtures before they :arevusedifor other purposw, ,ror example-lets fuel'gas, itzis not rusuallyneoessary or desirableto remove thecarhon dioxidatbeeau's'e thesegaseous .mixtures .have very high heating rvalues even withcarbon-dioxide i present. where- .iore selective removal .ofvhydrogensulfide elimi hates the extra expense of removing .carbonditoxide.

I have discovered that hydrogen sulfide may be selectively andinexpensively removed from mixtures containing the same and .caiibohdioxide by contacting suc'h .niix'tures with an aqueous suspensioncontaining a preeipitated iron nyaroxide incluuin'grer'rou's hydroxide.anahavin a H within therange 4 v. 7

The invention is particularly "well "adapted; "to gas purification in acyclic process 'wherein'the :aqueous treating suspension, 'in apurification stage, contacts "a "gaseous mixture and removes hydrogensulfide therefrom by preo'ipitation of iron sulfide, and, in aregeneration stage, "contaetsafree-oxygen containing as whichmenders itagain capable of purifying more of the gaseous "mixture, probablythrough oxidation :oi iron :sultide to iron hydroxide andfree sulfur.'frhe'ilatter 'mayb'e removed fromthetreating suspension by any of'themethods which are known inzthewlrt, es-subsequently"described.

t a een-.mundzthat aqueous. suspensions vof Aron mydroxide exhibttwhamcter-istiematterns mi 2 lbehavior when employed in cyclic processes.as deseribedabove. :Fonexample, the pilot .a freshly i-prepared:ferrous hydroxide suspension will decrease during the firstpurificationstage, probably because 20f neutralization of the.hydioxide, and :increase during subsequent purifioation stagesyprobablybecause-of reduction of ferric hydroxide :formed during the regenerationstage'tozthe more soluble ferrous hydroxide. The 51-! decreases :duringall regeneration stages, probably because ofxoxidationof ferrous:hydroxrue. The *eontraryapfl BfiGCtSzOf the-purification andregeneration stages nan be made to balance each other, withz'the resultthatincyclic operation the pl-iandgapparently, the composition-ofthe-regenerated treatingzagent are approximately the sameafterzeachmycle. A1so,:since thepI-Idecreases :during both the 'firstpurifi'cation stage and the .first regeneration stage, it is possible tobegin with -a freshly prepared ztreating agent "having apflsomewhattahovev'l :and, withoutv adding any acidic reagents, :endtthe.first and subsequent cycles with a regenerated treating agent having a[13H within the 2 range 4-7.

'The' aqueous treating suspensionzof :the invention'niay ibe prep'aredrm:a variety .of ways, :such 'asby adding a basicaoompound, .e-.;g,,sodium hydrox'ide, "sodium tetraborate, sodium acetate 1.01aii'i'moniiimihydroxide, to an aqueous solution of a-water-soluble;ironsalt isuch.asferIousnhIOride -'o'r"fer rous sulfatexhavingastronglyiacidio-anion. Afi' 'in'smuble preeipitateioflimnf hydroxide-.wi11'be -i0rnied in the solution. .If1ilh8 :other ions which enterunto the reaotion .do :not ';;form ra wateia-in soluble-salt,thehydroxideofzironsmay, byfiltration, be obtainedsubstantiallyireeoftother mate- ?rials"and-may be-*suspended;,inffreshwater v-to give 'a treating suspension =eontaining no :substantialamount 'of dissolved =material .other than the small amount-of ironhydroxide which 1 dissolves. -'The presence of substantial amounts ofother dissolved material in the treating suspension as I not, however,detrimental to the action thereof ..more than twice thatgiven bysodium(hydroxide,

that the pH of the treating suspension be within the range -7. Above apH of 7, carbon dioxide begins to be absorbed by the suspension to anundesirable degree.

The means employed to secure the desired pH- will vary according to thenature of the iron hydroxide suspension. If this suspension hasinitially a pH above 7, it may be reduced to an operable value byaddition of an acid or by agitation with a free oxygen-containing gas,thereby oxidizing part of the ferrous hydroxide to ferric hydroxide, acompound which is less soluble in water than ferrous hydroxide andimparts to the mixture a lower pH.

It is to be noted that an aqueous material containing in suspension agiven concentration of iron hydroxide will have a pH which is a functionof the degree of oxidation of the aqueous material, that is, of therelative proportions of ferric and ferrous hydroxides therein. Forexample, an aqueous suspension of iron hydroxide prepared by adding 0.4mole of ferrous sulfate and 0.1 mole of sodium hydroxide to a liter ofwater will, upon oxidation, decrease in pH from a value above 7 to avalue substantially below 4. o

.minimum pH is obtained when all ferrous hydroxide has been oxidized toferric hydroxide. For the purposes of the present invention, it ispreferred to work with treating agents contain"- ing only a minorproportion of ferric hydroxide, because at pH values which are too low,the possibilities of corrosion become too great, and also the amount ofhydrogen sulfide which the treating suspension can hold is considerablyreduced.

The concentration of iron in the treating suspension may vary.Concentrations of iron within the range 0.05 to 0.8 mole per liter havebeen found satisfactory. It is inadvisable to use concentrations above0.8 mole per liter, because the suspension then becomes too viscous tobe handled readily, and also excessive foaming is likely to take placewhen the treating suspension intimately contacts a gaseous material. Apreferred range is 0.3 to 0.5 mole per liter.

When the treating suspension is prepared by adding a. base to'an aqueoussolution of a watersoluble iron salt, the latter should have aconcentration in the solution within the range 0.05 to 0.8 mole perliter. The concentration of the added base may vary considerably. It isgenerally desirable toadd enough base to give the maximum amount ofhydroxide precipitate without obtaining as a consequence a treatingsuspension which cannot be conveniently brought within the desired pHrange by oxidation.

In general, the amount of basic compound should not be substantiallygreater than that required to precipitate all of the iron ion as ironhydroxide. When alkali metal hydroxide or alkali metal acetate is usedas the basic compound with a ferrous salt, the number of moles of thebasic compound added to a liter ofsolution should be at least 0.1 andnot substantially greater than twice the numberof moles of ferrous saltin the original solution. When alkali metal tetraborate is used as thebasic compound, the number of moles added to a liter of solution ispreferably within the range 0.05-0.1; when more than 0.1 mole is used,the suspension becomes viscous and difficult to handle.

The temperature at which the purification stage is carried out haslittle or no elTect on the amount of hydrogen sulfide removed. Byincreasing the temperature of regeneration, however, from, for example,70 F. to F., the rate of regeneration may be approximately doubled.

For purposes of illustration, an embodiment of the invention is shown onthe accompanying drawing, inwhich the figure is a diagrammatic view ofan apparatus for removing hydrogen sulfide from gaseous mixtures.

The impure gaseous mixture to be treated is treating tower 2. The gasflows upwardly through the treating. tower 2 where it is intimatelycontacted with a descending stream or spray of aqueous treating agent.The treated gas leaves the tower through an outlet I6 for storage or further use.

The treating agent is made up and its pH adjusted to a value within therange 4-1. The late ter may be done, as previously indicated, by puttinga freshly prepared ferrous hydroxide treat? ing agent having a pHsomewhat above 7 through one cycle of purification and regeneration, atthe end of which its pH is within the proper range 4-1. The treatingagent is introduced through lines 3 and 4 into the top of the treatingtower 2', wherein it descends, intimately contacting the gases to betreated, which are flowing upwardly through the tower. The fouledtreating agent is removed from the base of the treating tower ,2 throughline 5 and is conducted through line I9 to the top of the regenerationtower 6, where the treating agent is intimately contacted with afree-oxygen containing gas, e. g. air, for regeneration purposes.

The regeneration air in introduced through line 1 into the base of theregeneration tower 6. The air flows upwardly through the tower,intimately contacting the fouled treating agent which flows downwardlythrough the tower. The air is discharged from the tower into theatmosphere through line 8. The revivified treating agent is removed fromthe base of the regeneration tower 6 through line 9and passes eitherthrough line I 0 to sulfur removal apparatus II or through lines I2, I3, 20, and 4 to the top of treating tower 2 again.

In sulfur removal apparatus II, the treating agent contacts a solventwhich is selective to sulfur, e. g., benzene, naphtha, or other liquidhydrocarbon, introduced through line H. Conventional means are providedwithin the sulfur removal apparatus II for separation of the hydrocarbonand aqueous phases, and hydrocarbon containing sulfur dissolved thereinis removed through line I5 to solvent recovery means not shown. Thesulfur-free regenerated treating agent passes from sulfur removal tank II through lines I4, I3, 20, and 4 to the top of treating tower 2.

Insteadof a solvent extraction system as described above, sulfur removalapparatus I I may comprise any other type of apparatus capableofremoving free sulfur from admixture with iron hydroxide precipitates inan aqueous medium. For example, a sort of flotation method may be usedwherein .the regenerated treatmg-a e lt ris agitated agas underconditions such that the sulfur becomes" concentrated in: a* layer." offoam above the treating agent.

Since"; in the regeneration of the treating agent, it is. generallydesirableto obtain by oxidation onl'ya minor prop ortion" of ferrichydroxide" in the suspended iicnlhydroxideg'.andismcmiti is notalwayssconvenient to providein .regenerationto'wer. [l the close-controlover oxidizing, conditions that Wouldienahle obtaining for the. entirefoul'edtre'ating agentstream the exact degreeiof oxid'ationrequired;ittmay bedesira'ble insome caseslito pass only a; portion of" thefouled. treating." agent through line l9" to the regeneration tower: 5this portiomcambe oxidized completely to theferric hydrox-id'e.v stage,then recombined; through: lines 9;, ['21, and. I3, with theunregeneratedl portion passin through line I'll. When thisproc'edii'reiis followed, the unregenerat'ed portion will" usuallycontain some ferrous hydroxide which has notreacted'withrhydrogensulndeg inl'fact it is usually desirable :toproportioni-nthev flow ratesin :tr'eati'ng tower 2 in such a way thatthere isan excess. of iron hydroxide in the treating agent over thatrequired for reaction with hydrogen sulfide. The commingled regeneratedand unregenerated streams, passing through lines and 4 to the treatingtower 2, will usually therefore contain both ferrous and ferrichydroxides, even in the case where the regenerated stream has beencompletely oxidized to the ferric hydroxide state.

Either or both the treating tower 2 and the regeneration tower 6 can beof a type common in industrial usage. Any satisfactory method forinsuring intimate contacting of the gas and treating agent may beemployed, such for instance as spray nozzles, bubble cap plates, orpacking the towers with raschig rings or the like. The only requirementis that sufiicient contact time and surface be provided to effect thedesired results.

From the above description, it will be apparent that I have produced asimple and efiicient method for removing hydrogen sulfide from gaseousmixtures without substantially decreasing the carbon dioxide content ofthe mixtures. An important feature resides in controlling the pH of thesuspension employed in treating the gases in order to preventsubstantial absorption of carbon dioxide.

The following example illustrates the'invention:

Example Ferrous sulfate was dissolved in water to produce 0.4. molarsolution. 0.8 mole per liter of sodium hydroxide was added to thesolution to precipitate ferrous hydroxide. The resulting suspension hada pH of about 11.5, which was reduced to about 8 by the addition ofsulfuric acid.

An acidic gas containing hydrogen sulfide and carbon dioxide was bubbledthrough the treating suspension until the latter had absorbed about 11grams per liter of hydrogen sulfide and had a pH of about 5.75.

Air was bubbled through the fouled suspension to oxidize the ironsulfide therein. At the end of this regeneration stage, the pH of thesuspension was about 5.4.

Subsequent gas treatments raised the pH to about 5.75, and subsequentregenerations decreased it to about 5.4. Negligible amounts of carbondioxide were removed from the acidic gas during the treating stages.

It is to be noted that, in the preparation of the treating agent, theferrous hydroxide may be separated by filtration from the solution inwhich itt'issprecipitated'z andisuspendedin treshwatersta give a quitecomparable? treating; agent. contaim.- ing no: substantial;. amount-on?dissolved materiali The process: of: the; present; inventio1'r, whereinthe :treatihgragentzis. acidic: and the: foul'editreat-v in'gagentcontaining iron'rsulfiderislikewisezacidie; is" further: advantageousover processeswherein an". alkaline! suspension: of. iron: sulfide'i is:regen-- eratedaby oxidatiomin that-such regeneratiomis more; readilyaccomplished; in: an acidic medium' probably; because? therironasulfide:is; more soluble in: acidic: medium; at condition: which: favors the?reaction: or; the: iron sulfidei recipitate with oxygen; v

' Inzmy copending. application. Serial. No; 8.6;892; filediltprill 12,119.49; .thereis disclosed and. claimTd a process for selective removalof hydrogemsullfide from: ait-gaseouszmixturei containingtheasameandzcanboni dioxide by; treating. thagaseous:

1 ture with a treating: agentaconsistingi essentially ooiwa'teri and,-dissolved.theneiman ironsalt hav ingzaweakly*acidic;anion;.theatreatirigagentvmrw ing a pH withinlthesrange'A'li, I

-. Isolaimc;

1. Process for selectively removing H28 from gases also containing CO2which comprises: contacting in a treating zone a gas containing H28 andCO2 with a treating agent comprising an aqueous suspension of ferroushydroxide thereby to form iron sulfide; removing from said treating zonea gas containing substantially all of the CO2 originally present, butsubstantially less than all of the H28 originally present; separatelyremoving from said treating zone the fouled treating agent nowcontaining iron sulfide in suspension; regenerating the removed treatingagent by contact with free-oxygen-containing gas in a regeneration zone;re-introducing the regenerated treating agent into said treating zone;measuring the pH of the regenerated treating agent and adjusting saidpH, thereby to maintain the regenerated treating agent in acidiccondition and at a pH not less than 4 substantially throughout theduration of the process.

2. Process according to claim 1 wherein said treating agent ismaintained at a pH not less than 5.

3. Process according to claim 1 wherein said aqueous suspension contains0.05 to 0.8 mole of iron hydroxide per liter of suspension.

4. Process according to claim 1 wherein said aqueous suspension contains0.3 to 0.5 mole of iron hydroxide per liter of suspension.

5. Process according to claim 1 wherein said treating agent comprisesiron hydroxide consisting essentially of a major proportion of ferroushydroxide and a minor proportion of ferric hydroxide.

6. Process according to claim 1 wherein said gas containing H2S and CO2is a petroleum refinery fuel gas.

'7. Process according to claim 1 wherein free sulfur is separated fromsaid regenerated treating agent prior to re-introduction of the latterinto said treating zone.

8. Process for selectively removing HzS from gases also containing CO2which comprises: contacting in a treating zone a gas containing H25 andCO2 with a treating agent comprising an aqueous suspension of ferroushydroxide, said treating agent having been originally prepared byreacting an aqueous solution of a ferrous salt with a basic compoundselected from the group consisting of alkali metal and ammonium basiccompounds to precipitate ferrous hydroxide and but substantially lessthan all of the HzS original- 5 1y present; separately removing fromsaid treating zone the fouled treating agent now containing iron sulfidein suspension; regenerating the removed treating agent by contact withfree-oxygen-containing gas in a regeneration zone; reintroducing theregenerated treating agent into said treating zone; measuring the pH ofthe regenerated "treating agent and adjusting said pH, thereby tomaintain the regenerated treating agent in acidic condition and at a pHnot less than 4, substantially throughout the duration of the process.

- 9. Process according to claim 8 wherein said ferrous salt is ferroussulfate and said basic compound is an alkali metal hydroxide.

10. Process accordin to claim 8 wherein said ferrous salt is ferroussulfate and said basic compound is an alkali metal tetraborate.

11. Process according to claim 8 wherein said ferrous salt is ferroussulfate and said basic compound is an alkali metal acetate.

, KOERNER E. LEUTZ.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,510,342 Rambush Sept. 30, 1924 1,700,698 Fulweiler Jan. 29,1929 1,708,590 Reeson Apr. 9, 1929 1,847,795 Thorsell Mar. 1, 19321,854,491 Sperr Apr. 19, 1932 1,995,545 Leahy Mar. 26, 1935 2,028,125Shaw Jan. 14, 1936 2,085,523 Belchetz June 29, 1937 2,143,393 UlrichJan. 10, 1939 2,616,834 .Leutz Nov. 4, 1952 OTHER REFERENCES 7 Fe'ld:Article in Zeitschrift fiir angewandte Chemie, vol. 24, pages 97-102(pp. 99 and 100 pertinent), 1911.

1. PROCESS FOR SELECTIVELY REMOVING H2S FROM GASES ALSO CONTAINING CO2WHICH COMPRISES: CONTACTING IN A TREATING ZONE A GAS CONTAINING H2S ANDCO2 WITH A TREATING AGENT COMPRISING AN AQUEOUS SUSPENSION OF FERROUSHYDROXIDE THEREBY TO FORM IRON SULFIDE; REMOVING FROM SAID TREATING ZONEA GAS CONTAINING SUBSTANTIALLY ALL OF THE CO2 ORIGINALLY PRESENT, BUTSUBSTANTIALLY LESS THAN ALL OF THE H2S ORIGINALLY PRESENT; SEPARATELYREMOVING FROM SAID TREATING ZONE THE FOULED TREATING AGENT NOWCONTAINING IRON SULFIDE IN SUSPENSION; REGENERATING THE REMOVED TREATINGAGENT BY CONTACT WITH FREE-OXYGEN-CONTAINING GAS IN A REGENERATION ZONE;RE-INTRODUCING THE REGENERATED TREATING AGENT INTO SAID TREATING ZONE;MEASURING THE PH OF THE REGENERATED TREATING AGENT AND ADJUSTING SAIDPH, THEREBY TO MAINTAIN THE REGENERATED TREATING AGENT IN ACIDICCONDITION AND AT A PH NOT LESS THAN 4 SUBSTANTIALLY THROUGHOUT THEDURATION OF THE PROCESS.